The use of bacterial biopesticides such as Bacillus thuringiensis (Bt) is a viable alternative for insect control in agriculture and other areas (i.e. disease vectors) that will intensify the crop production in an economically sustainable and environmental friendly way. The Bt Cry proteins are highly specific, harmless to humans, vertebrates and plants, and are completely biodegradable so no residual toxic products accumulate in the environment (Schnepf et al., 1998). To date, over 200 cry genes sequences have been determined and classified in 44 families and different subclasses (Crickmore et al., 1998, 2005). Additionally, Bt produces a number of extracellular compounds that might contribute to virulence as phospholipases, proteases, chitinases and other toxins as β-exotoxin or VIP proteins (Schnepf et al., 1998).
Despite extensive research over the last decades, only a few bacterial insecticidal toxins are used on a wide scale against the most damaging insect pests in biological pest control applications, such as those using Bt-plants.
The S-layer is an ordered structure of proteinaceous paracrystalline array, which cover the surface of many archaea and eubacteria (Beveridge et al., 1997; Sara and Sleytr, 2000) and can constitute up to 15% of total cell protein. The function of S-layer proteins has not been accurately defined, but it has been proposed that these proteins are involved in cell integrity and shape maintenance. Also, it has been hypothesized that they may be involved in macromolecular exchange with the environment since they are the outermost cell envelope component (Beveridge et al., 1997). In some gram-negative pathogenic bacteria, they have been implicated in virulence and resistance to complement-mediated killing (Sara and SLeytr, 2000; Pei and Blaser, 1990). In B. cereus, the S-layer has been described to promote interactions with human leucocytes and with the host, contributing to the pathogenicity (Kotiranta et al., 1998). In B. anthracis it has been proposed that the S-layer and the capsule might cooperate in the interaction with the host (Mignot et al., 2002).
In B. anthracis two different S-Layer proteins (SAP and EA1) have been described (Mignot et al., 2002). The presence of these proteins is not required for normal encapsulation of the Bacilli (Mesnage et al., 1998). These proteins appear sequentially in a growth phase-dependent manner, with the synthesis of SAP preceding that of EA1 (Mignot et al., 2002). In B. thuringiensis subsp. galleria an S-Layer protein was described, SlpA, that is similar to the SAP of B. anthracis. The S-layer CTC protein was described in B. thuringiensis subsp. finitimus (GenBank accession number AAR23791), this protein is similar to EA1 from B. anthracis (Sun et al., 2001). CTC has a molecular size of 100 kDa and forms parasporal bodies during the sporulation phase of growth.
Xu et al. (2004) describe the presence of a 120 kDa protein in an SDS-PAGE analysis of a crystal/spore mixture of a sporulating Bacillus thuringiensis strain. Supernatant of this crystal/spore mixture, obtained after dissolution, centrifugation and dialysis, was found to prolong the survival of mice injected with an infectious blood sample of Plasmodium berghei. The N-terminal sequence (15 amino acids) of the 120 kDa protein showed 100% homology to that of the S-layer protein of Bacillus thuringiensis subsp. galleria (reported in Mesnage et al., 1998). The nucleotide sequence of the gene encoding this protein is not provided but is said to encode 821 amino acid residues with a deduced molecular weight of 87.5 kDa. No isolated or purified protein, nor a recombinant host producing this protein, was tested for its efficacy against Plasmodium infection herein. Also, the strain from which such crystal/spore mixture was isolated has not been deposited or specifically described in this paper.